FIELD OF THE INVENTION
The invention relates to a blocking-oscillator switched-mode power supply for sinusoidal current consumption, having a semiconductor switching element for the clocked application of an alternating voltage, rectified by a bridge rectifier and smoothed by a smoothing capacitor, to a primary winding of a transformer, and having a control device for triggering the semiconductor switching element.
Switched-mode power supplies are high-frequency sources of interference that require spark-type interference suppression. If switched-mode power supplies are supplied with mains-type alternating voltage that needs to be rectified, then undesirable harmonic currents appear in the power supply system or power grid, and in the future such harmonic currents will no longer be accepted by power supply enterprises, because limit values of allowable harmonic currents are being made more stringent. A sinusoidal current consumption by the switched-mode power supplies is therefore sought.
While heretofore in switched-mode power supplies, spark interference suppression functioned essentially well as a result of current-compensated chokes, nevertheless the harmonic currents caused by the switched-mode power supplies remained unnoticed in the grid. Since the more stringent limit values for allowable harmonic currents also apply to switched-mode power supplies in communications equipment, such as TV sets, it is necessary to reduce the harmonic currents. That can be achieved only by means of a sinusoidal current consumption by the switched-mode power supply.
Blocking-oscillator switched-mode power supplies with sinusoidal current consumption have already been described. Published European Application No. 0 464 240 A1, corresponding to U.S. application Ser. No. 08/108,771, filed Aug. 18, 1993, for instance, for the sinusoidal current consumption by the switched-mode power supply, has proposed the clocked application to the primary winding of a transformer of an alternating voltage, rectified by a bridge rectifier, but unsmoothed, through a semiconductor switching element. The semiconductor switching element is triggered by a control device in accordance with at least one primary current signal to be applied to a first input terminal of the control device. That primary current signal is furnished through a one-way rectifier configuration, connected to the bridge rectifier and followed by an RC element.
A switched-mode power supply shown in German Published, Non-Prosecuted Application DE 40 08 652 A1 has a charging capacitor, which assures in cooperation with an inductive resistor and a diode network that brief drops in or outages of the mains alternating voltage will be compensated for. To that end, the capacitor is charged when the switching element is turned off and discharged when the switching element is conducting.